Many studies and reports have been made for increasing the heat transfer from a horizontal tube to a fluid therearound as a fluidized bed. The authors report overall heat transfer coefficients of 35 BTU/hr .degree.F. ft.sup.2. They also report that values in the range of 80-120 BTU/hr .degree.F. ft.sup.2 are theoretically possible. Studies reveal the cause of the low coefficients to be a defluidized pile of catalyst on the top of the tube and a stagnant gas pocket on the bottom of the tube, both insulating the tube partially, as illustrated in FIG. 1 of the drawings.
The high coke yields expected from new proposed catalytic cracking of residue will require additional heat removal facilities from the regenerator for unit heat balance. The more heat required to be carried off, the number and size of coils are required to be increased.
Instead, it would be desirable to increase the efficiency of heal transfer through the heat exchanger tubes and thus decrease the number and size of heat exchanger coils. Also, it would be desirable to have heat transfer coefficient in the 100 BTU/hr .degree.F. ft.sup.2 range which would be three times that with the conventional circular cross-section.